TEM observations of iron and nickel foils irradiated by MeV fullerenes at room temperature

It is shown that a new specific effect takes place in iron during irradiati on with similar to 10 MeV fullerene beams. A dislocation line with various shapes is observed in the close vicinity of the projectile path leading to a strain field confined inside a 20-40 nm diameter "cylinder". Similarly to previous observations in titanium and zirconium (Dammak et al., Phys. Rev. Lett. 74 (1995) 1135), this effect might be due to the strong localisation of the energy deposited during the projectile slowing-down process. Howeve r no damage is observable in nickel irradiated in the same conditions. Thes e results should be explained by the previously established relation betwee n the sensitivity of a metal to a high rate of energy deposition in electro nic excitations and the existence of a high pressure phase in the pressure- temperature phase diagram (Dammak et al., Nucl. Instr. and Meth. B 107 (199 6) 204). (C) 1998 Elsevier Science B.V. All rights reserved.